Polyimide resins having improved electrical and mechanical properties have been utilized as insulating protective coatings for electronic parts. Since most polyimide resins are insoluble in organic solvents, polyimide protective coatings are generally prepared by using a polyamic acid solution which is a polyimide precursor, and applying the solution to substrates, followed by heat curing. This conventional approach, however, has several problems that the polyamic acid solution is rather difficult to apply because of its high viscosity, heat curing requires high temperatures in excess of 300.degree. C., and the resulting polyimide coatings show poor adherence to such substrates as nickel, aluminum, silicon, and silicon oxide films. A number of solutions to these problems have been proposed.
For improving the adherence to substrates, Japanese Patent Publication Nos. 27439/1968 and 7213/1984 propose polyimide-siloxane copolymers in which a diamine component, which is a reactant for forming polyimide, is partially replaced by a siloxane-containing diamine. Also, Japanese Patent Publication Nos. 32162/1983, 32163/1983, 266436/1986, 207438/1986, and 29510/1989 disclose a method for mixing or reacting a polyamic acid or polyimide precursor with a silane having an amino group or acid anhydride group. However, the former method had the problem that copolymers with an increased content of siloxane showed poor heat resistance. In the latter method, the polyamic acid solution experienced a substantial loss of shelf stability with an increasing amount of silane added.
In order to avoid the heat treatment at as high temperatures as beyond 300.degree. C. which is necessary in curing polyamic acid into polyimide films, it was proposed to form films directly from a polyimide resin by dissolving a polyimide resin having a siloxane bond itself in a suitable solvent (see Japanese Patent Publication Nos. 83228/1986, 118424/1986, and 118425/1986) or by using an imide resin having an alkoxysilyl group (see Japanese Patent Publication Nos. 35076/1980 and 35077/1980). Both the methods left unacceptable problems in practical applications. Polyimide resin films resulting from the former method had poor solvent resistance by nature. The latter method required to add water for hydrolysis of alkoxysilane, but storage stability after hydrolysis was poor.